3. Atom is made up of three
particles:electrons,protons and
neutrons.
Protons and neutrons are present in
a nucleus at the centre of the atom.
Electrons are outside the nucleus.
4. The existence of electrons in an atom was shown by J.J
Thomson in 1897.Thomson passed electricity at high voltage
through a gas at very low pressure taken in a discharge tube.
Streams of minute particles were given out by the cathode.
These steams of particles are called cathode rays. Cathode
rays consist of small, negatively charged particles called
ELECTRONS. An electron is usually represented by the
symbol ‘e-’.
5. The existence of proton in an atom was shown by
E.Goldstein. When Goldstein passed electricity at high
voltage through a gas at very low pressure taken in a
discharge tube, streams of heavy particles were given out by
the anode. These streams of particles are called anode rays.
Anode rays consist of positively charged particles called
PROTONS.A proton is usually represented by ‘p’.
6. The existence of neutron in an atom was shown by James Chadwick in 1932. The
neutron is a neutral particle found in the nucleus of an atom. Atoms of all the
elements contain neutrons except ordinary hydrogen atom which does not
contain any neutron. A neutron is represented by the symbol ‘n’.
7. When J.J Thomson proposed his model of the atom in 1903, then only electrons
and protons were known to be present in the atom. According to Thomson’s
model of the atom :
1. An atom consists of a sphere of positive charge with negatively charged
electrons embedded in it.
2. The positive and negative charges in an atom are equal in magnitude, due to
which an atom is electrically neutral. It has no overall positive or negative
charge.
8.
9. It was Rutherford’s alpha particle scattering experiment which led to the discovery
of a small positively charged nucleus in the atom containing all the protons and
neutrons. When fast moving alpha particle are allowed to strike a very thin gold foil
in vacuum, it is found that:
1. Most of the alpha particles pass straight through the gold foil without any
deflection from their original path.
2. A few alpha particles are deflected through small angles and a few are deflected
through large angles.
3. A very few alpha particles completely rebound on hitting the gold foil and turn
back on their path.
11. DRAWBACKS OF RUTHERFORD’S MODEL OF
THE ATOM
A major drawback of Rutherford's model of the atom is that it does not explain the
stability of the atom.
12. The present concept of atom was given by Neils Bohr. The Bohr’s model of atom
can be described as:
1. An atom is made up of three particles : electrons, protons and neutrons.
2. The protons and neutrons are located in a small nucleus at centre of the atom.
3. The electrons revolve rapidly round the nucleus in fixed paths called energy
levels or shells.
4. There is a limit to the number of electrons which each energy level can hold.
5. Each energy level is associated with a fixed amount of energy.
6. There is no change in the energy of electrons as long as they keep revolving in
the same energy level, and the atom remains stable.
13.
14. The number of protons in one atom of an element is known as atomic number
of that element.
Atomic number of = Number of protons in
an element one atom of element
The atomic number of an element is denoted by the letter ‘Z’.
The atomic number of an element tells us two things:
1. It tells us the number of protons in one atom of the element.
2. It tells us the number of electrons in one normal atom of the element.
15. The total number of protons and neutrons present in one atom of an element is
known as its mass number.
Mass number = No. of protons + No. of neutrons
The mass number of an element is denoted by the letter ‘A’.
16. Isotopes are atoms of the same element having the same atomic number but
different mass numbers.
Isotopes of hydrogen:
e e e
p P p
n 2n
ORDINARY HYDROGEN HEAVY HYDROGEN VERY HEAVY HYDROGEN
ATOMIC MASS =1u ATOMIC MASS =2u ATOMIC MASS= 3u
17. Isobars are the atoms of different elements having different atomic numbers but
the same mass numbers.
ISOBAR PROTONS NEUTRONS MASS NUMBER
Ar 18 22 18+22=40
Ca 20 20 20+20=40
